1. Field of the Invention
This invention relates to a center pivot irrigation system having the ability to irrigate the corners of the field being irrigated. More particularly, this invention relates to an improved means for controlling a center pivot last regular drive unit and corner span.
2. Description of the Related Art
Conventional center pivot irrigation systems comprise an elongated main boom pivotally connected at its inner end to a center pivot structure and extending outwardly therefrom. The main boom of the conventional center pivot irrigation system is comprised of a plurality of pipes connected together in an end-to-end fashion which are supported upon a plurality of drive towers. In most conventional center pivot irrigation systems, the outermost or last regular drive tower or unit is the master tower with the other drive towers being selectively driven in response to angular displacement of the boom section adjacent thereto.
In the early 1970s, corner systems for center pivot irrigation systems were developed to enable the corners of a square field to be irrigated. See, for example, U.S. Pat. Nos. 3,797,517; 3,802,627; and 3,902,668. Corner systems usually consist of an extension boom or arm, sometimes referred to as a corner span, which is pivotally connected to the outer end of the main boom and which is supported on at least one steerable drive tower. A guidance system is provided for guiding or steering the extension tower, and extension boom, out into the corners of the field as the main arm travels around the field. Perhaps the most popular method of guiding the extension tower is the buried wire system described in U.S. Pat. No. 3,902,668. However, many different types of systems for driving the wheels of the extension tower have been previously provided. See, for example, U.S. Pat. Nos. 4,508,269 and 4,674,681. Further, in assignee's current production model, relative movement between the main boom and the extension boom is sensed with the drive wheels on the steerable drive tower on the extension boom being driven when the main boom has moved forwardly a predetermined distance with respect to the extension boom. When the extension boom "catches up" with the main arm, the driving of the drive wheels on the extension tower is discontinued. This process is continuously repeated during the movement of the system through the field.
Prior to the instant invention, the corner span relied upon a speed control to maintain position with the last regular drive unit as discussed hereinabove. Therefore, the corner span was required to maintain a slight angle with the center pivot portion of the machine in order to control the joint position. The inability of the corner span to completely extend resulted in less area being irrigated.